Does this cube have a ledge around the top? If not, have you filled it up with water yet? The reason I ask is that it may bow some on the sides. If it does, cut some acrylic strips (about 2" wide) and glue them around the top perpendicular to the sides. That will stiffen the top edge enough to stop most of the bow. Just a thought.
Logan J

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Life is not measured by the number of breaths we take, but by the moments that take our breath away.
Logan J